1. Field of the Invention
The present invention relates to a digital-to-analog converter useful in, for example, a circuit that drives a liquid crystal display.
2. Description of the Related Art
With the recent increase in the size of liquid crystal display devices, various needs have arisen for improved performance in their driving circuits. One need is for a gradation scale with more gradation levels, especially for the display of more vivid colors. The current state of the art is a liquid crystal display device that can reproduce over one billion different colors by using ten bits of data (1024 gradation levels) for each of the three primaries (red, green, blue). The increased number of gradation levels demands improved performance from the digital-to-analog (D/A) converters that convert digital signals received from an outside source to analog signals. D/A converters of the resistor string type are often employed.
The simplest resistor string D/A converters have the structure shown in FIG. 3, which converts two-bit digital data (bits 1D and 2D and their complementary values 1DB and 2DB), and FIG. 4, which converts three-bit digital data (bits 1D-3D and their complementary values 1DB-3DB). An output decoder comprising transistor switches selects one of the voltage levels (V0, V1, V2, . . . ) generated by the resistor string (R1, R2, . . . ) for output (Vout). With this circuit configuration, each time the number of bits increases by one, the number of resistors and transistors substantially doubles, doubling the circuit area.
Japanese Patent Application Publication No. 2000-183747 (U.S. Pat. No. 6,373,419) describes an alternative circuit configuration with fewer resistors and transistors, but the output decoder requires an averaging voltage-follower amplifier with two parallel differential input stages, an arrangement that consumes an undesirably large amount of current.
Japanese Patent Application Publication No. 62-024713 describes a different circuit configuration in which the number of transistors and resistors increases more slowly with the number of bits, but this configuration tends to produce voltage level fluctuations when several hundred output decoders are connected in parallel to the same resistor string, as is the case in circuits for driving large liquid crystal displays.